This quantitative study will compare the costs and complications of two radiotherapy (RT) techniques used for early invasive breast cancer and ductal carcinoma in situ (DCIS). Our goals are to elucidate the risks, complication rates, and costs of RT and subsequent complications for whole breast RT compared to brachytherapy-based accelerated partial breast RT, a newer form of RT that may be associated with new or increased risks of morbidity;that lacks long-term clinical trial evidence of effectiveness;and that may have some advantages not borne out due to limited follow-up thus far. By using Medicare claims data for women ages 66+ with a breast cancer diagnosis and treated with breast- conserving surgery and RT, we will evaluate whether there are different rates of complications and differential cost burdens of these two RT techniques. Our specific aims are to: 1) Identify and classify related and possibly-related complications of whole breast RT and brachytherapy-based accelerated partial breast RT within one year of RT;2) Calculate and compare absolute numbers and rates of RT complications by RT modality, accounting for patient and healthcare system factors;and 3) Assess and compare the costs of the RT modalities, accounting for patient and healthcare system factors. This is the first known study to evaluate costs and complication rates of brachytherapy-based breast RT compared to whole breast RT on a population level. Thus far, only small, selected studies have outlined complication rates of brachytherapy-based breast RT. Such small sample sizes are likely to miss rarer events and could underestimate actual complication rates. Likewise, costs have only been evaluated on the basis of expected Medicare reimbursement for treatment itself, but not for subsequent problems associated with treatment, such as unexpected explantation or reimplantation of a balloon catheter, problematic seromas, fat necrosis, or infection. We will assess costs for treatment and complications within one year of treatment. Our novel methods will minimize endogeneity bias by RT type. This study will allow us to identify and quantify the explicit health trade-offs and cost impact to the healthcare system for women undergoing RT following breast- conserving surgery on a population-based level. This is particularly important for this new technology that has higher up-front costs with no known additional therapeutic benefit compared to the standard of care. PUBLIC HEALTH RELEVANCE: This quantitative study will compare the costs and complications of two radiotherapy techniques used for early invasive breast cancer and ductal carcinoma in situ (DCIS). Our goals are to elucidate the risks, complication rates, and costs of radiotherapy and subsequent complications for whole breast radiotherapy compared to the much-newer and less-studied accelerated partial breast radiotherapy, using Medicare claims data for women ages 66+. By integrating information on radiotherapy efficacy, effectiveness, variations in practice patterns, and care quality, we can guide medical decision making so that physicians and patients can choose optimal clinical interventions for the treatment of breast cancer.